It is frequently desirable to reinforce rubber articles (such as, tires, conveyor belts, power transmission belts, timing belts and hoses) by incorporating therein steel reinforcing elements. Pneumatic vehicle tires are often reinforced with cords prepared from brass-coated steel filaments. Such tire cords are frequently composed of high carbon steel or high carbon steel coated with a thin layer of brass. Such a tire cord can be a monofilament, but normally is prepared from several filaments which are stranded together. In most instances, depending upon the type of tire being reinforced, the strands of filaments are further cabled to form the tire cord.
It is important for the steel alloy utilized in filaments for reinforcing elements to exhibit high strength and ductility as well as high fatigue resistance. Unfortunately, many alloys which possess this demanding combination of requisite properties cannot be processed in a practical commercial operation. More specifically, it is extremely impractical to patent many such alloys which otherwise exhibit extremely good physical properties because they have a slow rate of isothermal transformation which requires a long period in the soak zone (transformation zone). In other words, in the patenting process a long time period in the transformation zone is required to change the microstructure of the steel alloy from face-centered cubic to body-centered cubic.
In commercial operations, it is desirable for the transformation from a face-centered cubic microstructure to a body-centered cubic microstructure in the transformation phase of the patenting process to occur as rapidly as possible. The faster the rate of transformation, the less demanding the equipment requirements are at a given throughput. In other words, if more time is required for the transformation to occur, then the length of the transformation zone must be increased to maintain the same level of throughput. It is, of course, also possible to reduce throughputs to accommodate for the low rate of transformation by increasing the residence time in the transformation zone (soak). For these reasons, it is very apparent that it would be desirable to develop a steel alloy having a fast rate of isothermal transformation in patenting which also exhibits high strength, high ductility and high fatigue resistance.
The patenting process is a heat treatment applied to steel rod and wire having a carbon content of 0.25 percent or higher. The typical steel for tire reinforcement usually contains about 0.65 to 0.85 percent carbon, 0.5 to 0.7 percent manganese and 0.15 to 0.5 percent silicon, with the balance of course being iron. The object of patenting is to obtain a structure which combines high tensile strength with high ductility, and thus impart to the wire the ability to withstand a large reduction in area to produce the desired finished sizes possessing a combination of high tensile strength and good toughness.
Patenting is normally conducted as a continuous process and typically consists of first heating the alloy to a temperature within the range of about 850.degree. C. to about 1150.degree. C. to form austenite, and then cooling at a rapid rate to a lower temperature at which transformation occurs which changes the microstructure from face-centered cubic to body-centered cubic and which yields the desired mechanical properties. In many cases, while it is desired to form a single allotrope, a mixture of allotropes having more than one microstructure is, in fact, produced.
U.S. Pat. No. 4,960,473, U.S. Pat. No. 5,066,455, U.S. Pat. No. 5,167,727 and U.S. Pat. No. 5,229,069 relate to steel alloys for use in manufacturing reinforcing wires for rubber products, such as tires, which can be patented in a low-cost process due to their having a very fast rate of isothermal transformation. These patents disclose the realization of tensile strengths of up to 3256 MPa. However, the tire industry is now calling for even higher tensile strength. In fact, tensile strengths in the range of 4000 MPa to 5000 MPa would be desirable. Even greater tensile strengths of over 4500 MPa are deemed to be more desirable.